1. Field of the Invention
This invention relates to light metering devices, and, more particularly, to light metering devices for single lens reflex cameras.
2. Description of the Prior Art
In the past, the light metering devices using a light beam splitting member, for example, a micro beam splitter formed in, for example, the condenser lens so that light is directed to the photosensitive element, or a Fresnel lens positioned on the exit face of the pentagonal roof type prism to concentrate light, have been proposed. These two types had drawbacks that the light distribution over the field of view of the finder was degraded, or when altering the light metering mode, it was necessary to interchange the light beam splitting member.
FIGS. 1 and 2 show an example of each of the conventional types of light metering devices. In FIG. 1, light coming from an object to be photographed and entering through a photographic lens 1 of the single lens reflex camera is reflected upward by a reflex mirror 2 and passes through a focusing screen 3, a light beam splitting member 4, a photosensitive element 5, a condenser lens 6, and a pentagonal prism 7 to an eyepiece lens 8. In this type, when the object is observed through the eyepiece lens 8, that part of the light which travels through the light beam splitting member 4 is shut out. Thus, there is produced a bad effect of casting a shadow on the central portion of the viewfield of the finder. Also the light metering modes of the camera are the center-weighted light metering with a heavy emphasis on the central portion of the area of the image format, and the average light metering mode with uniform emphasis on the overall area of the field. Because the region of the light rays to be taken in the photosensitive element 5 is determined by the shape of the light beam splitting member 4, when it is desired to change the light metering area, the condenser lens 6, having the light beam splitting member 4 formed therein, must be replaced.
In the conventional example of FIG. 2, the object light emerging from the photographic lens is reflected by the reflex mirror 2 to a focusing screen 9 and further directed through the pentagonal prism 7 to the eyepiece lens 8. Meanwhile, by means of a Fresnel lens 10 positioned on the exit face of the pentagonal prism 7, light is concentrated on the photosensitive element 5 confronting the exit face. In this case, though the central portion of the viewfield of the finder is not darkened as in the case of FIG. 1, it is very difficult to selectively operate many light metering modes such as those mentioned above.